I am trying to calculate the resistor to limit the base current going from the pin to the BC817-40. The USBizi pin is at 3.3V with a max current of 4mA per pin.
Current check: The min hfe of the transistor is 250 with a fully on Ic of 500mA so this should be 2mA from each pin to fully turn it on. I need 32 transistors connected so max package current needs to be kept to a minimum so this would be 64mA so should be fine.
Voltage check: The USBizi runs at 3.3V. The transistor Vbe is 1.2V so this should be fine as the datasheet for the LPC2387 says a minimum of 2.5V when high.
So according to this Rb = (3.3-1.2)/0.002 = 1050Ohm so pick next nearest below = 1k
Would this work? As I say it just doesn’t feel right…
Thanks in advance and my apologies for being a bit dumb on this one.
Note that that mos has a Vgs(th) of 2V, but that means that it STARTS conducting at 2V. Study Figure 7. At Vgs of 3V you can expect a drain current of max ±70mA. Their stated drain current of 475mA is at a Vgs of 10V.
Thanks for the input, it is very much appreciated.
If I went with this MOS I shouldn’t need a limiting resistor between the USBizi and gate pin should I? (Assuming this as there is only 700pF of Input Capacitance)
It’s not mentioned what is being driven, or the voltage and current levels needed. There are multiple high side or low side driver chips available. Up to eight outputs per chip. Usually include under and over voltage, thermal protection and over current protection. Look at Infineon and ST Micro.